1. Field of the Invention
The present invention relates to a molded semiconductor device and a method for fabricating the same. More particularly, the present invention relates to a semiconductor die assembly utilizing a multi-part lead frame having dissimilar materials and the method for fabricating the same. The multi-part lead frame can be used for a wide variety of types of lead frames, such as modified conventional lead frames, leads-over-chip (LOC) lead frames, hybrid lead frames, etc.
2. State of the Art
Conventional well known molded semiconductor devices are constructed by assembling and interconnecting a semiconductor device to a lead frame and molding the structure in plastic. In a xe2x80x9cconventionalxe2x80x9d or xe2x80x9ctraditionalxe2x80x9d type of lead frame construction, a lead frame is made from a metal ribbon with each lead frame including a paddle (also known as a die paddle, die-attach tab, or island) for attaching a semiconductor device thereto and a plurality of leads arranged such that the leads do not overlap the paddle on which the semiconductor device is to be mounted.
In order to overcome inherent limitations created by the size and bond pad arrangement of semiconductor device assemblies using conventional types of lead frames, leads-over-chip (LOC) semiconductor device assemblies have been employed. The LOC lead frame configuration for a semiconductor device replaces the conventional lead frame configuration with a lead frame configuration having no die paddle and having lead fingers or leads that extend over the active surface of the semiconductor device. The semiconductor device is supported by being adhesively secured to the lead fingers by means of a dielectric film disposed between the undersides of a portion of the lead fingers and the semiconductor device. Examples of assemblies implementing LOC lead frame technology are disclosed in U.S. Pat. Nos. 5,184,208; 5,252,853; 5,286,679; 5,304,842; and 5,461,255. In some instances, LOC lead frame assemblies employ additional quantities of adhesive to enhance physical support of the semiconductor device for handling.
Traditional lead frame semiconductor device assemblies have a semiconductor device attached to a die paddle of the lead frame. The die paddle having a semiconductor device attached thereto is located adjacent the inner ends of the lead fingers of the lead frame so that the inner ends of the lead fingers are in close lateral proximity to the bond pads located at the periphery of the active surface of the semiconductor device. Wire bonds are formed between the inner ends of the lead fingers and the bond pads on the periphery of the semiconductor device.
In contrast, LOC lead frame assemblies have lead fingers of the lead frame extending over the active surface of the semiconductor device and adhesively attached thereto. This permits physical support of the semiconductor device from the lead fingers themselves, permits more diverse placement of the bond pads on the active surface of the semiconductor device, and permits the use of the lead fingers for heat transfer from the semiconductor device. However, use of LOC lead frame assemblies in combination with plastic packaging of the LOC lead frame assembly has demonstrated some shortcomings of LOC technology and economics.
After wire bonding the semiconductor device to the lead fingers of the lead frame forming an assembly, the most common manner of forming a plastic package about a semiconductor device assembly is transfer molding. In the transfer molding of an LOC type lead frame and semiconductor device assembly, a semiconductor device, which is adhesively suspended by its active surface from the lead fingers of an LOC lead frame and has the bond pads of the semiconductor device and the inner ends of lead fingers of the lead frame connected by wire bonds, is placed in a mold cavity and molded in a thermosetting polymer to form a highly cross-linked matrix.
One of the technological shortcomings of the prior art LOC semiconductor device assemblies is that the adhesive tape used to bond to the lead fingers of the lead frame does not adequately lock the lead fingers in position. In some instances, the adhesive on the tape is not strong enough to lock the lead fingers in position for wire bonding as the lead fingers may pull away from the tape before wire bonding. Alternately, the lead fingers may pull away from the tape after wire bonding of the semiconductor device but before molding of the semiconductor device and LOC lead frame thereby either causing shorts between adjacent wire bonds or the wire bonds to pull loose from either the bond pads of the semiconductor device or the lead fingers of the lead frame. With respect to economic considerations, a cost reduction can be realized by replacing the more expensive adhesives and tapes used in the LOC lead frame and semiconductor device assembly with a lower cost lead frame having characteristics of both a conventional type lead frame configuration and an LOC type lead frame configuration.
An alternative type lead frame to an LOC lead frame and semiconductor device assembly is disclosed in U.S. Pat. No. 4,984,059 to Kubota et al. In this alternative type lead frame and semiconductor device assembly, two metal lead frames are used. A die paddle, onto which a semiconductor device is subsequently attached, is formed between the longitudinal sides of a first lead frame. A second lead frame is formed having lead fingers extending between the longitudinal sides thereof. An assembly is formed by welding the first lead frame having a semiconductor device attached to the die paddle to the second lead frame having the lead fingers thereof extending over the active surface of the semiconductor device. The welding is accomplished by welding cradles running along the two longitudinal sides of each lead frame. Alignment of the two lead frames is accomplished by matching alignment holes found on the cradles with alignment holes in the longitudinal sides of each lead frame. The double lead frame assembly thus eliminates the need for tapes or adhesives as a means to support the die from the lead fingers themselves as the semiconductor device is supported by the die paddle of the first lead frame. In an alternative arrangement is the ""059 patent, a semiconductor device is attached to a die paddle having arms extending therefrom with the arms of the die paddle being attached to receiving portions of a lead frame having a plurality of leads formed therewith. However, use of either double lead frame assemblies or separately formed die paddles subsequently attached to receiving portions of a lead frame in combination with a molded packaging lead frame assembly so formed has demonstrated shortcomings in terms of technology and economics.
One such shortcoming involves the manufacturing area. In the molding process, the double lead frame process requires molds specifically adapted for receiving two lead frames. Thus, in order to practice the double lead frame process, existing xe2x80x9csingle lead framexe2x80x9d equipment must be replaced.
Another shortcoming affects the design and reliability of the packaged semiconductor device. The double lead frame assemblies disclosed in the prior art are limited to use of metal ribbons of the same material to form both lead frame structures.
A shortcoming of the separately formed die paddle subsequently attached to receiving portions of a lead frame is that the separately formed die paddle is difficult to handle and to accurately attach to the lead frame, thereby creating wire bonding problems between the leads of the lead frame and the bond pads of the semiconductor device.
However, designing double lead frame assemblies that utilize different metallic and/or non-metallic materials to fabricate the two lead frames allows packaging and operational advantages. Materials can be selected which closely match either the mold compound properties, the semiconductor device properties or both in order to capitalize on a desired effect or characteristic (e.g. fabricating a die paddle with A-42 type alloy material to deal with thermal expansion and fabricating the lead frame with copper material to increase speed of transmission). Additionally, packaging advantages can be realized by using materials of different thicknesses to obtain desired effects such as conservation of space to form smaller packages or increased heat dissipation from the package. Furthermore, desirable characteristics of different types of lead frames may be combined into double lead frame assemblies, particularly where the semiconductor device is accurately located with respect to the lead frame.
From the foregoing, the prior art has neither provided a multilayer molded semiconductor package that is fabricated through conventional single lead frame assembly and molding processes, nor has it provided for use of dissimilar lead frame materials to fabricate a multilayer molded plastic semiconductor package.
The present invention provides a multi-part lead frame and semiconductor device assembly which includes a die paddle, the multi-part lead frame being separately formed and assembled from dissimilar or separate materials, if desired. The use of separate or different materials for the lead fingers of the lead frame and die paddle provides packaging and operational advantages through the availability of a variety of materials which can be selected to closely match the mold compound and semiconductor device properties. Another advantage of the present invention is to provide a semiconductor device assembly fabricated from less expensive materials than those currently being used.
These and other advantages of the present invention are accomplished by a semiconductor device or die assembly that includes a semiconductor device having an active surface having, in turn, a plurality of bond pads formed thereon and a lead frame assembly including a first lead frame and a second lead frame. The first lead frame includes a die paddle onto which the semiconductor device is attached, first carriers and tie bars connecting the die paddle to the carriers of the lead frame. The first carriers are usually vertically spaced from the die paddle such that the die paddle is located in a horizontal plane below the first carriers so that when the semiconductor die is mounted on the die paddle, the active surface of the die is located in substantially the same horizontal plane as the first carriers. The second lead frame of the lead frame assembly includes a plurality of lead fingers extending inwardly from second carriers having second alignment holes therein, each lead finger of the plurality of lead fingers including an inner lead portion and an outer lead portion secured to a carrier. The inner lead portion of each lead finger is located in a predetermined location with respect to the bonds pads on the active surface of a semiconductor device attached to the die paddle of the first lead frame after portions of the first and second lead frames have been joined. The inner lead portion of each lead finger is also horizontally spaced at a predetermined location from the bond pads located on the active surface of the semiconductor device attached to the die paddle of the first lead frame after portions of the first and second lead frames have been joined. Wire bonds interconnect the inner lead portion of each lead finger and the bond pads of the semiconductor die. The second lead frame further includes tab receiving portions for securing or attaching portions of tie bars of the first lead frame to the second lead frame.
The present invention further includes a method of fabricating a semiconductor device. In accordance with the method of the present invention, a semiconductor device having an active surface having, in turn, bond pads formed thereon is used. Also used in the method of the present invention is a first lead frame including a die paddle, first carriers having alignment holes therein, and tie bars having cut zones and tabs therein. The tie bar interconnects the die paddle and first carriers. The semiconductor device is attached to the die paddle. Next, a second lead frame having second carriers and a plurality of lead fingers is used in the method of the present invention. The second carriers of the second lead frame include tab receiving portions affixed thereto and second alignment members. The first and second lead frames are aligned by aligning the plurality of first alignment members and second alignment members together. The first and second lead frames are then joined by securing the tabs of the first lead frame to the tab receiving portions of the second lead frame. The tie bars are then cut at a point between the tabs and the first carriers and the first carriers are discarded. Finally, the inner lead ends of the lead fingers and the bond pads of the semiconductor device are interconnected with wire bonds.
The semiconductor device assembly of the present invention permits the use of xe2x80x9csingle lead framexe2x80x9d equipment through all existing manufacturing steps instead of replacing existing equipment for a so called xe2x80x9cdouble lead framexe2x80x9d process.
The semiconductor device assembly may be molded into the desired package using conventional apparatus and methods.
Securing of the first and second lead frames (or portions thereof) may be accomplished by welding, adhesive bonding, or any other suitable method of bonding.
The lead frame configuration of the present invention may be a conventional type, an LOC type, a hybrid type, etc.